PhD Thesis (PDF) - Department of Astronomy - University of Virginia

Fig. 3.11.— Top: Cumulative X-ray spectrum of the resolved sources with more than
3 σ detected flux in the entire field of NGC 1600. The spectrum has 1 σ error bars
and is overlaid by the solid histogram of the best-fit model spectrum (Table 3.2).
Bottom: Contribution to χ 2 with the sign indicating the sign of the residual.
thermal bremsstrahlung model with kTh = 4.73 keV (Table 3.2, row [1]) or a power-
law model with a photon number spectral index of Γ = 1.76 (row [3]). The fits were
not improved significantly when the absorbing column was allowed to vary (rows [2]
and [4]), so we fixed the hydrogen column at the Galactic value, NH = 4.86 × 10 20
cm −2 (Dickey & Lockman 1990). Since the sources were very luminous, we also
attempted a disk blackbody model; however, its χ 2 was worse by ∼ 20 for the same
number of dof. Finally, we attempted a disk blackbody+power-law spectral model
(kTdisk = 1.44, Γ = 1.95, power-law responsible for ∼70% of flux). This model had
a χ 2 lower than our bremsstrahlung or power-law fits by ∼2; however, the dof was
also reduced by 2. In addition, this fit had an unconstrained disk temperature and
a poorly constrained power law. We could have adopted either the bremsstrahlung
or the power-law model for our best fit. For comparison with other papers and since
the power law was constrained more tightly than the bremsstrahlung temperature,
104